Configuring a Basic BGP Network

Transcription

1 Configuring a Basic BGP Network Last Updated: October 19, 2011 This module describes the basic tasks to configure a basic Border Gateway Protocol (BGP) network. BGP is an interdomain routing protocol that is designed to provide loop-free routing between organizations. The Cisco IOS implementation of the neighbor and address family commands is explained. This module also contains tasks to configure and customize BGP peers, implement BGP route aggregation, configure BGP route origination, and define BGP backdoor routes. BGP peer group definition is documented, peer session templates are introduced, and update groups are explained, Finding Feature Information, page 1 Prerequisites for Configuring a Basic BGP Network, page 1 Restrictions for Configuring a Basic BGP Network, page 2 Information About Configuring a Basic BGP Network, page 2 How to Configure a Basic BGP Network, page 17 Configuration Examples for a Basic BGP Network, page 83 Where to Go Next, page 96 Additional References, page 96 Feature Information for Configuring a Basic BGP Network, page 98 Finding Feature Information Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document. Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to An account on Cisco.com is not required. Prerequisites for Configuring a Basic BGP Network Before configuring a basic BGP network, you should be familiar with the "Cisco BGP Overview" module. Americas Headquarters: Cisco Systems, Inc., 170 West Tasman Drive, San Jose, CA USA

2 Restrictions for Configuring a Basic BGP Network BGP Version 4 Restrictions for Configuring a Basic BGP Network A router that runs Cisco IOS software can be configured to run only one BGP routing process and to be a member of only one BGP autonomous system. However, a BGP routing process and autonomous system can support multiple address family configurations. Information About Configuring a Basic BGP Network BGP Version 4, page 2 BGP Router ID, page 3 BGP-Speaker and Peer Relationships, page 3 BGP Autonomous System Number Formats, page 3 Cisco Implementation of 4-Byte Autonomous System Numbers, page 6 BGP Peer Session Establishment, page 6 Cisco Implementation of BGP Global and Address Family Configuration Commands, page 7 BGP Session Reset, page 9 BGP Route Aggregation, page 9 BGP Aggregation Route AS-SET Information Generation, page 9 Routing Policy Change Management, page 10 Conditional BGP Route Injection, page 11 BGP Peer Groups, page 12 BGP Backdoor Routes, page 12 Peer Groups and BGP Update Messages, page 13 BGP Update Group, page 13 BGP Dynamic Update Group Configuration, page 13 BGP Peer Templates, page 13 Inheritance in Peer Templates, page 14 Peer Session Templates, page 15 Peer Policy Templates, page 16 BGP IPv6 Neighbor Activation Under the IPv4 Address Family, page 17 BGP Version 4 Border Gateway Protocol (BGP) is an interdomain routing protocol designed to provide loop-free routing between separate routing domains that contain independent routing policies (autonomous systems). The Cisco IOSsoftware implementation of BGP version 4 includes multiprotocol extensions to allow BGP to carry routing information for IP multicast routes and multiple Layer 3 protocol address families including IP Version 4 (IPv4), IP Version 6 (IPv6), and Virtual Private Networks version 4 (VPNv4). BGP is mainly used to connect a local network to an external network to gain access to the Internet or to connect to other organizations. When connecting to an external organization, external BGP (ebgp) peering sessions are created. Although BGP is referred to as an exterior gateway protocol (EGP) many networks within an organization are becoming so complex that BGP can be used to simplify the internal network used within the organization. BGP peers within the same organization exchange routing information through internal BGP (ibgp) peering sessions. 2

3 BGP Router ID Information About Configuring a Basic BGP Network BGP requires more configuration than other routing protocols, and the effects of any configuration changes must be fully understood. Incorrect configuration can create routing loops and negatively impact normal network operation. BGP Router ID BGP uses a router ID to identify BGP-speaking peers. The BGP router ID is a 32-bit value that is often represented by an IPv4 address. By default, the Cisco IOS software sets the router ID to the IPv4 address of a loopback interface on the router. If no loopback interface is configured on the router, then the software chooses the highest IPv4 address configured to a physical interface on the router to represent the BGP router ID. The BGP router ID must be unique to the BGP peers in a network. BGP-Speaker and Peer Relationships A BGP-speaking router does not discover another BGP-speaking device automatically. A network administrator usually manually configures the relationships between BGP-speaking routers. A peer device is a BGP-speaking router that has an active TCP connection to another BGP-speaking device. This relationship between BGP devices is often referred to as a neighbor but, as this can imply the idea that the BGP devices are directly connected with no other router in between, the term neighbor will be avoided whenever possible in this document. A BGP speaker is the local router and a peer is any other BGPspeaking network device. When a TCP connection is established between peers, each BGP peer initially exchanges all its routes--the complete BGP routing table--with the other peer. After this initial exchange only incremental updates are sent when there has been a topology change in the network, or when a routing policy has been implemented or modified. In the periods of inactivity between these updates, peers exchange special messages called keepalives. A BGP autonomous system is a network controlled by a single technical administration entity. Peer routers are called external peers when they are in different autonomous systems and internal peers when they are in the same autonomous system. Usually, external peers are adjacent and share a subnet; internal peers may be anywhere in the same autonomous system. For more details about external BGP peers, see the "Connecting to a Service Provider Using External BGP" module. For more details about internal BGP peers, see the "Configuring Internal BGP Features" module. BGP Autonomous System Number Formats Prior to January 2009, BGP autonomous system numbers that were allocated to companies were 2-octet numbers in the range from 1 to as described in RFC 4271, A Border Gateway Protocol 4 (BGP-4). Due to increased demand for autonomous system numbers, the Internet Assigned Number Authority (IANA) will start in January 2009 to allocate four-octet autonomous system numbers in the range from to RFC 5396, Textual Representation of Autonomous System (AS) Numbers, documents three methods of representing autonomous system numbers. Cisco has implemented the following two methods: Asplain--Decimal value notation where both 2-byte and 4-byte autonomous system numbers are represented by their decimal value. For example, is a 2-byte autonomous system number and is a 4-byte autonomous system number. Asdot--Autonomous system dot notation where 2-byte autonomous system numbers are represented by their decimal value and 4-byte autonomous system numbers are represented by a dot notation. For 3

4 Information About Configuring a Basic BGP Network BGP Autonomous System Number Formats example, is a 2-byte autonomous system number and is a 4-byte autonomous system number (this is dot notation for the decimal number). For details about the third method of representing autonomous system numbers, see RFC Asdot Only Autonomous System Number Formatting In Cisco IOS Release 12.0(32)S12, 12.4(24)T, and later releases, the 4-octet (4-byte) autonomous system numbers are entered and displayed only in asdot notation, for example, 1.10 or When using regular expressions to match 4-byte autonomous system numbers the asdot format includes a period which is a special character in regular expressions. A backslash must be entered before the period (for example, 1\.14) to ensure the regular expression match does not fail. The table below shows the format in which 2- byte and 4-byte autonomous system numbers are configured, matched in regular expressions, and displayed in show command output in Cisco IOS images where only asdot formatting is available. Table 1 Asdot Only 4-Byte Autonomous System Number Format Format Configuration Format Show Command Output and Regular Expression Match Format asdot 2-byte: 1 to byte: 1.0 to byte: 1 to byte: 1.0 to Asplain as Default Autonomous System Number Formatting In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, and later releases, the Cisco implementation of 4-byte autonomous system numbers uses asplain as the default display format for autonomous system numbers, but you can configure 4-byte autonomous system numbers in both the asplain and asdot format. In addition, the default format for matching 4-byte autonomous system numbers in regular expressions is asplain, so you must ensure that any regular expressions to match 4-byte autonomous system numbers are written in the asplain format. If you want to change the default show command output to display 4-byte autonomous system numbers in the asdot format, use the bgp asnotation dot command under router configuration mode. When the asdot format is enabled as the default, any regular expressions to match 4-byte autonomous system numbers must be written using the asdot format, or the regular expression match will fail. The tables below show that although you can configure 4-byte autonomous system numbers in either asplain or asdot format, only one format is used to display show command output and control 4-byte autonomous system number matching for regular expressions, and the default is asplain format. To display 4-byte autonomous system numbers in show command output and to control matching for regular expressions in the asdot format, you must configure the bgp asnotation dot command. After enabling the bgp asnotation dot command, a hard reset must be initiated for all BGP sessions by entering the clear ip bgp * command. If you are upgrading to an image that supports 4-byte autonomous system numbers, you can still use 2-byte autonomous system numbers. The show command output and regular expression match are not changed and remain in asplain (decimal value) format for 2-byte autonomous system numbers regardless of the format configured for 4-byte autonomous system numbers. 4

5 BGP Autonomous System Number Formats Information About Configuring a Basic BGP Network Table 2 Default Asplain 4-Byte Autonomous System Number Format Format Configuration Format Show Command Output and Regular Expression Match Format asplain 2-byte: 1 to byte: to byte: 1 to byte: to asdot 2-byte: 1 to byte: 1.0 to byte: 1 to byte: to Table 3 Asdot 4-Byte Autonomous System Number Format Format Configuration Format Show Command Output and Regular Expression Match Format asplain 2-byte: 1 to byte: to byte: 1 to byte: 1.0 to asdot 2-byte: 1 to byte: 1.0 to byte: 1 to byte: 1.0 to Reserved and Private Autonomous System Numbers In Cisco IOS Release 12.0(32)S12, 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, 12.4(24)T, and later releases, the Cisco implementation of BGP supports RFC RFC 4893 was developed to allow BGP to support a gradual transition from 2-byte autonomous system numbers to 4-byte autonomous system numbers. A new reserved (private) autonomous system number, 23456, was created by RFC 4893 and this number cannot be configured as an autonomous system number in the Cisco IOS CLI. RFC 5398, Autonomous System (AS) Number Reservation for Documentation Use, describes new reserved autonomous system numbers for documentation purposes. Use of the reserved numbers allow configuration examples to be accurately documented and avoids conflict with production networks if these configurations are literally copied. The reserved numbers are documented in the IANA autonomous system number registry. Reserved 2-byte autonomous system numbers are in the contiguous block, to and reserved 4-byte autonomous system numbers are from to inclusive. Private 2-byte autonomous system numbers are still valid in the range from to with being reserved for special use. Private autonomous system numbers can be used for internal routing domains but must be translated for traffic that is routed out to the Internet. BGP should not be configured to advertise private autonomous system numbers to external networks. Cisco IOS software does not remove private autonomous system numbers from routing updates by default. We recommend that ISPs filter private autonomous system numbers. Autonomous system number assignment for public and private networks is governed by the IANA. For information about autonomous-system numbers, including reserved number assignment, or to apply to register an autonomous system number, see the following URL: 5

6 Information About Configuring a Basic BGP Network Cisco Implementation of 4-Byte Autonomous System Numbers Cisco Implementation of 4-Byte Autonomous System Numbers In Cisco IOS Release 12.0(32)SY8, 12.0(33)S3, 12.2(33)SRE, 12.2(33)XNE, 12.2(33)SXI1, and later releases, the Cisco implementation of 4-byte autonomous system numbers uses asplain for example--as the default regular expression match and output display format for autonomous system numbers, but you can configure 4-byte autonomous system numbers in both the asplain format and the asdot format as described in RFC To change the default regular expression match and output display of 4-byte autonomous system numbers to asdot format, use the bgp asnotation dot command followed by the clear ip bgp * command to perform a hard reset of all current BGP sessions. In Cisco IOS Release 12.0(32)S12, and 12.4(24)T, the Cisco implementation of 4-byte autonomous system numbers uses asdot--1.2 for example--as the only configuration format, regular expression match, and output display, with no asplain support. For an example of BGP peers in two autonomous systems using 4- byte numbers, see the figure below. To view a configuration example of the configuration between three neighbor peers in separate 4-byte autonomous systems configured using asdot notation, see the Examples Configuring a BGP Routing Process and Peers Using 4-Byte Autonomous System Numbers, page 84. Cisco also supports RFC 4893, which was developed to allow BGP to support a gradual transition from 2- byte autonomous system numbers to 4-byte autonomous system numbers. To ensure a smooth transition, we recommend that all BGP speakers within an autonomous system that is identified using a 4-byte autonomous system number be upgraded to support 4-byte autonomous system numbers. A new private autonomous system number, 23456, was created by RFC 4893, and this number cannot be configured as an autonomous system number in the Cisco IOS CLI. Figure 1 BGP Peers in Two Autonomous Systems Using 4-Byte Numbers BGP Peer Session Establishment When a BGP routing process establishes a peering session with a peer it goes through the following state changes: 6

7 Cisco Implementation of BGP Global and Address Family Configuration Commands Information About Configuring a Basic BGP Network Idle--Initial state the BGP routing process enters when the routing process is enabled or when the router is reset. In this state, the router waits for a start event, such as a peering configuration with a remote peer. After the router receives a TCP connection request from a remote peer, the router initiates another start event to wait for a timer before starting a TCP connection to a remote peer. If the router is reset then the peer is reset and the BGP routing process returns to the Idle state. Connect--The BGP routing process detects that a peer is trying to establish a TCP session with the local BGP speaker. Active--In this state, the BGP routing process tries to establish a TCP session with a peer router using the ConnectRetry timer. Start events are ignored while the BGP routing process is in the Active state. If the BGP routing process is reconfigured or if an error occurs, the BGP routing process will release system resources and return to an Idle state. OpenSent--The TCP connection is established and the BGP routing process sends an OPEN message to the remote peer, and transitions to the OpenSent state. The BGP routing process can receive other OPEN messages in this state. If the connection fails, the BGP routing process transitions to the Active state. OpenReceive--The BGP routing process receives the OPEN message from the remote peer and waits for an initial keepalive message from the remote peer. When a keepalive message is received, the BGP routing process transitions to the Established state. If a notification message is received, the BGP routing process transitions to the Idle state. If an error or configuration change occurs that affects the peering session, the BGP routing process sends a notification message with the Finite State Machine (FSM) error code and then transitions to the Idle state. Established--The initial keepalive is received from the remote peer. Peering is now established with the remote neighbor and the BGP routing process starts exchanging update message with the remote peer. The hold timer restarts when an update or keepalive message is received. If the BGP process receives an error notification, it will transition to the Idle state. Cisco Implementation of BGP Global and Address Family Configuration Commands The address family model for configuring BGP is based on splitting apart the configuration for each address family. All commands that are independent of the address family are grouped together at the beginning (highest level) of the configuration, and these are followed by separate submodes for commands specific to each address family (with the exception that commands relating to IPv4 unicast can also be entered at the beginning of the configuration). When a network operator configures BGP, the flow of BGP configuration categories is represented by the following bullets in order: Global configuration--configuration that is applied to BGP in general, rather than to specific neighbors. For example, the network, redistribute, and bgp bestpath commands. Address family-dependent configuration--configuration that applies to a specific address family such as policy on an individual neighbor. The relationship between BGP global and BGP address family-dependent configuration categories is shown in the table below. Table 4 Relationships Between BGP Configuration Categories BGP Configuration Category Global address family-independent Configuration Sets Within Category One set of global address family-independent configurations 7

8 Information About Configuring a Basic BGP Network Cisco Implementation of BGP Global and Address Family Configuration Commands BGP Configuration Category Address family-dependent Configuration Sets Within Category One set of global address family-dependent configurations per address family Address family configuration must be entered within the address family submode to which it applies. The following is an example of BGP configuration statements showing the grouping of global address family-independent and address family-dependent commands. router bgp <AS>! AF independent part neighbor <ip-address> <command>! Session config; AF independent address-family ipv4 unicast! AF dependant part neighbor <ip-address> <command>! Policy config; AF dependant exit-address-family address-family ipv4 multicast! AF dependant part neighbor <ip-address> <command>! Policy config; AF dependant exit-address-family address-family ipv4 unicast vrf <vrf-name>! VRF specific AS independent commands! VRF specific AS dependant commands neighbor <ip-address> <command>! Session config; AF independent neighbor <ip-address> <command>! Policy config; AF dependant exit-address-family The following example shows actual BGP commands that match the BGP configuration statements in the previous example: router bgp router-id bgp log-neighbor-changes neighbor remote-as neighbor remote-as address-family ipv4 unicast neighbor activate network mask exit-address-family address-family ipv4 multicast neighbor activate neighbor advertisement-interval 25 network mask exit-address-family address-family ipv4 vrf vpn1 neighbor activate network mask exit-address-family In Cisco IOS Releases 12.0(22)S, 12.2(15)T, and later releases, the bgp upgrade-cli command simplifies the migration of BGP networks and existing configurations from the network layer reachability information (NLRI) format to the address family format. Network operators can configure commands in the address family identifier (AFI) format and save these command configurations to existing NLRI formatted configurations. The BGP hybrid command-line interface (CLI) does not add support for complete AFI and NLRI integration because of the limitations of the NLRI format. For complete support of AFI commands and features, we recommend upgrading existing NLRI configurations with the bgp upgrade-cli command. For a configuration example of migrating BGP configurations from the NLRI format to the address family format, see. 8

9 BGP Session Reset Information About Configuring a Basic BGP Network BGP Session Reset Whenever there is a change in the routing policy due to a configuration change, BGP peering sessions must be reset using the clear ip bgp command. Cisco IOS software support the following three mechanisms to reset BGP peering sessions: Hard reset--a hard reset tears down the specified peering sessions including the TCP connection and deletes routes coming from the specified peer. Soft reset--a soft reset uses stored prefix information to reconfigure and activate BGP routing tables without tearing down existing peering sessions. Soft reconfiguration uses stored update information, at the cost of additional memory for storing the updates, to allow you to apply new BGP policy without disrupting the network. Soft reconfiguration can be configured for inbound or outbound sessions. Dynamic inbound soft reset--the route refresh capability, as defined in RFC 2918, allows the local router to reset inbound routing tables dynamically by exchanging route refresh requests to supporting peers. The route refresh capability does not store update information locally for non disruptive policy changes. It instead relies on dynamic exchange with supporting peers. Route refresh must first be advertised through BGP capability negotiation between peers. All BGP routers must support the route refresh capability. To determine if a BGP router supports this capability, use the show ip bgp neighborscommand. The following message is displayed in the output when the router supports the route refresh capability: Received route refresh capability from peer. The bgp soft-reconfig-backup command was introduced to configure BGP to perform inbound soft reconfiguration for peers that do not support the route refresh capability. The configuration of this command allows you to configure BGP to store updates (soft reconfiguration) only as necessary. Peers that support the route refresh capability are unaffected by the configuration of this command. BGP Route Aggregation BGP peers store and exchange routing information and the amount of routing information increases as more BGP speakers are configured. The use of route aggregation reduces the amount of information involved. Aggregation is the process of combining the attributes of several different routes so that only a single route is advertised. Aggregate prefixes use the classless interdomain routing (CIDR) principle to combine contiguous networks into one classless set of IP addresses that can be summarized in routing tables. Fewer routes now need to be advertised. Two methods are available in BGP to implement route aggregation. You can redistribute an aggregated route into BGP or you can use a form of conditional aggregation. Basic route redistribution involves creating an aggregate route and then redistributing the routes into BGP. Conditional aggregation involves creating an aggregate route and then advertising or suppressing the advertising of certain routes on the basis of route maps, autonomous system set path (AS-SET) information, or summary information. The bgp suppress-inactive command configures BGP to not advertise inactive routes to any BGP peer. A BGP routing process can advertise routes that are not installed in the routing information database (RIB) to BGP peers by default. A route that is not installed into the RIB is an inactive route. Inactive route advertisement can occur, for example, when routes are advertised through common route aggregation. Inactive route advertisements can be suppressed to provide more consistent data forwarding. BGP Aggregation Route AS-SET Information Generation AS-SET information can be generated when BGP routes are aggregated using the aggregate-address command. The path advertised for such a route is an AS-SET consisting of all the elements, including the 9

10 Information About Configuring a Basic BGP Network Routing Policy Change Management communities, contained in all the paths that are being summarized. If the AS-PATHs to be aggregated are identical, only the AS-PATH is advertised. The ATOMIC-AGGREGATE attribute, set by default for the aggregate-address command, is not added to the AS-SET. Routing Policy Change Management Routing policies for a peer include all the configurations for elements such as route map, distribute list, prefix list, and filter list that may impact inbound or outbound routing table updates. Whenever there is a change in the routing policy, the BGP session must be soft cleared, or soft reset, for the new policy to take effect. Performing inbound reset enables the new inbound policy configured on the router to take effect. Performing outbound reset causes the new local outbound policy configured on the router to take effect without resetting the BGP session. As a new set of updates is sent during outbound policy reset, a new inbound policy of the neighbor can also take effect. This means that after changing inbound policy you must do an inbound reset on the local router or an outbound reset on the peer router. Outbound policy changes require an outbound reset on the local router or an inbound reset on the peer router. There are two types of reset: hard reset and soft reset. The table below lists their advantages and disadvantages. Table 5 Advantages and Disadvantages of Hard and Soft Resets Type of Reset Advantages Disadvantages Hard reset No memory overhead. The prefixes in the BGP, IP, and Forwarding Information Base (FIB) tables provided by the neighbor are lost. Not recommended. Outbound soft reset Dynamic inbound soft reset No configuration, no storing of routing table updates. Does not clear the BGP session and cache. Does not require storing of routing table updates, and has no memory overhead. Does not reset inbound routing table updates. Both BGP routers must support the route refresh capability (in Cisco IOS Release 12.1 and later releases). Does not reset outbound routing table updates. 10

11 Conditional BGP Route Injection Information About Configuring a Basic BGP Network Type of Reset Advantages Disadvantages Configured inbound soft reset (uses the neighbor softreconfiguration router configuration command) Can be used when both BGP routers do not support the automatic route refresh capability. In Cisco IOS Release 12.3(14)T, the bgp soft-reconfig-backup command was introduced to configure inbound soft reconfiguration for peers that do not support the route refresh capability. Requires preconfiguration. Stores all received (inbound) routing policy updates without modification; is memoryintensive. Recommended only when absolutely necessary, such as when both BGP routers do not support the automatic route refresh capability. Does not reset outbound routing table updates. Once you have defined two routers to be BGP neighbors, they will form a BGP connection and exchange routing information. If you subsequently change a BGP filter, weight, distance, version, or timer, or make a similar configuration change, you must reset BGP connections for the configuration change to take effect. A soft reset updates the routing table for inbound and outbound routing updates. Cisco IOS Release 12.1 and later releases support soft reset without any prior configuration. This soft reset allows the dynamic exchange of route refresh requests and routing information between BGP routers, and the subsequent readvertisement of the respective outbound routing table. There are two types of soft reset: When soft reset is used to generate inbound updates from a neighbor, it is called dynamic inbound soft reset. When soft reset is used to send a new set of updates to a neighbor, it is called outbound soft reset. To use soft reset without preconfiguration, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when the peers establish a TCP session. Routers running Cisco IOS releases prior to Release 12.1 do not support the route refresh capability and must clear the BGP session using the neighbor soft-reconfiguration router configuration command. Clearing the BGP session in this way will have a negative impact upon network operations and should be used only as a last resort. Conditional BGP Route Injection Routes that are advertised through the BGP are commonly aggregated to minimize the number of routes that are used and reduce the size of global routing tables. However, common route aggregation can obscure more specific routing information that is more accurate but not necessary to forward packets to their destinations. Routing accuracy is obscured by common route aggregation because a prefix that represents multiple addresses or hosts over a large topological area cannot be accurately reflected in a single route. Cisco IOS software provides several methods in which you can originate a prefix into BGP. The existing methods include redistribution and using the network or aggregate-address command. These methods assume the existence of more specific routing information (matching the route to be originated) in either the routing table or the BGP table. BGP conditional route injection allows you to originate a prefix into a BGP routing table without the corresponding match. This feature allows more specific routes to be generated based on administrative policy or traffic engineering information in order to provide more specific control over the forwarding of packets to these more specific routes, which are injected into the BGP routing table only if the configured conditions are met. Enabling this feature will allow you to improve the accuracy of common route aggregation by conditionally injecting or replacing less specific prefixes with more specific prefixes. Only 11

12 Information About Configuring a Basic BGP Network BGP Peer Groups prefixes that are equal to or more specific than the original prefix may be injected. BGP conditional route injection is enabled with the bgp inject-map exist-mapcommand and uses two route maps (inject map and exist map) to install one (or more) more specific prefixes into a BGP routing table. The exist map specifies the prefixes that the BGP speaker will track. The inject map defines the prefixes that will be created and installed into the local BGP table. BGP Peer Groups BGP Backdoor Routes Often, in a BGP network, many neighbors are configured with the same update policies (that is, the same outbound route maps, distribute lists, filter lists, update source, and so on). Neighbors with the same update policies can be grouped into BGP peer groups to simplify configuration and, more importantly, to make configuration updates more efficient. When you have many peers, this approach is highly recommended. In a BGP network topology with two border routers using ebgp to communicate to a number of different autonomous systems, using ebgp to communicate between the two border routers may not be the most efficient routing method. In the figure below, Router B as a BGP speaker will receive a route to Router D through ebgp, but this route will traverse at least two autonomous systems. Router B and Router D are also connected through an Enhanced Interior Gateway Routing Protocol (EIGRP) network (any IGP can be used here) and this route has a shorter path. EIGRP routes, however, have a default administrative distance of 90 and ebgp routes have a default administrative distance of 20, so BGP will prefer the ebgp route. Changing the default administrative distances is not recommended because changing the administrative distance may lead to routing loops. To cause BGP to prefer the EIGRP route, you can use the network backdoor command. BGP treats the network specified by the network backdoor command as a locally assigned network, except that it does not advertise the specified network in BGP updates. In the figure below, this means that Router B will communicate to Router D using the shorter EIGRP route instead of the longer ebgp route. Figure 2 BGP Backdoor Route Topology 12

13 Peer Groups and BGP Update Messages Information About Configuring a Basic BGP Network Peer Groups and BGP Update Messages BGP Update Group In Cisco IOS software releases prior to Release 12.0(24)S, 12.2(18)S, or 12.3(4)T, BGP update messages were grouped based on peer group configurations. This method of grouping neighbors for BGP update message generation reduced the amount of system processing resources needed to scan the routing table. This method, however, had the following limitations: All neighbors that shared peer group configuration also had to share outbound routing policies. All neighbors had to belong to the same peer group and address family. Neighbors configured in different address families could not belong to different peer groups. These limitations existed to balance optimal update generation and replication against peer group configuration. These limitations could cause the network operator to configure smaller peer groups, which reduced the efficiency of update message generation and limited the scalability of neighbor configuration. The introduction of the BGP (dynamic) update group in Cisco IOS Releases 12.0(24)S, 12.2(18)S, 12.3(4)T, or 12.2(27)SBC, provides a different type of BGP peer grouping from existing BGP peer groups. Existing peer groups are not affected but peers with the same outbound policy configured that are not members of a current peer group can be grouped into an update group. The members of this update group will use the same update generation engine. When BGP update groups are configured an algorithm dynamically calculates the BGP update group membership based on outbound policies. Optimal BGP update message generation occurs automatically and independently. BGP neighbor configuration is no longer restricted by outbound routing policies, and update groups can belong to different address families. BGP Dynamic Update Group Configuration In Cisco IOS Release 12.0(24)S, 12.2(18)S, 12.3(4)T, 12.2(27)SBC, and later releases, a new algorithm was introduced that dynamically calculates and optimizes update groups of neighbors that share the same outbound policies and can share the same update messages. No configuration is required to enable the BGP dynamic update group and the algorithm runs automatically. When a change to outbound policy occurs, the router automatically recalculates update group memberships and applies the changes by triggering an outbound soft reset after a 1-minute timer expires. This behavior is designed to provide the network operator with time to change the configuration if a mistake is made. You can manually enable an outbound soft reset before the timer expires by entering the clear ip bgp ip-address soft outcommand. In Cisco IOS Release 12.0(22)S, 12.2(14)S, 12.3(2)T, and prior releases, the update group recalculation delay timer is set to 3 minutes. For the best optimization of BGP update group generation, we recommend that the network operator keeps outbound routing policy the same for neighbors that have similar outbound policies. BGP Peer Templates To address some of the limitations of peer groups such as configuration management, BGP peer templates were introduced to support the BGP update group configuration. A peer template is a configuration pattern that can be applied to neighbors that share policies. Peer templates are reusable and support inheritance, which allows the network operator to group and apply distinct neighbor configurations for BGP neighbors that share policies. Peer templates also allow the 13

14 Information About Configuring a Basic BGP Network Inheritance in Peer Templates network operator to define very complex configuration patterns through the capability of a peer template to inherit a configuration from another peer template. There are two types of peer templates: Peer session templates are used to group and apply the configuration of general session commands that are common to all address family and NLRI configuration modes. Peer policy templates are used to group and apply the configuration of commands that are applied within specific address families and NLRI configuration modes. Peer templates improve the flexibility and enhance the capability of neighbor configuration. Peer templates also provide an alternative to peer group configuration and overcome some limitations of peer groups. BGP peer routers using peer templates also benefit from automatic update group configuration. With the configuration of the BGP peer templates and the support of the BGP dynamic update peer groups, the network operator no longer needs to configure peer groups in BGP and the network can benefit from improved configuration flexibility and faster convergence. A BGP neighbor cannot be configured to work with both peer groups and peer templates. A BGP neighbor can be configured to belong only to a peer group or to inherit policies from peer templates. The following restrictions apply to the peer policy templates: A peer policy template can directly or indirectly inherit up to eight peer policy templates. A BGP neighbor cannot be configured to work with both peer groups and peer templates. A BGP neighbor can be configured to belong only to a peer group or to inherit policies only from peer templates. Inheritance in Peer Templates The inheritance capability is a key component of peer template operation. Inheritance in a peer template is similar to node and tree structures commonly found in general computing, for example, file and directory trees. A peer template can directly or indirectly inherit the configuration from another peer template. The directly inherited peer template represents the tree in the structure. The indirectly inherited peer template represents a node in the tree. Because each node also supports inheritance, branches can be created that apply the configurations of all indirectly inherited peer templates within a chain back to the directly inherited peer template or the source of the tree. This structure eliminates the need to repeat configuration statements that are commonly reapplied to groups of neighbors because common configuration statements can be applied once and then indirectly inherited by peer templates that are applied to neighbor groups with common configurations. Configuration statements that are duplicated separately within a node and a tree are filtered out at the source of the tree by the directly inherited template. A directly inherited template will overwrite any indirectly inherited statements that are duplicated in the directly inherited template. Inheritance expands the scalability and flexibility of neighbor configuration by allowing you to chain together peer templates configurations to create simple configurations that inherit common configuration statements or complex configurations that apply very specific configuration statements along with common inherited configurations. Specific details about configuring inheritance in peer session templates and peer policy templates are provided in the following sections. When BGP neighbors use inherited peer templates it can be difficult to determine which policies are associated with a specific template. In Cisco IOS 12.0(25)S, 12.4(11)T, 12.2(33)SRB, 12.2(33)SB, and later releases, the detail keyword was added to the show ip bgp template peer-policy command to display the detailed configuration of local and inherited policies associated with a specific template. 14

15 Peer Session Templates Information About Configuring a Basic BGP Network Peer Session Templates Peer session templates are used to group and apply the configuration of general session commands to groups of neighbors that share session configuration elements. General session commands that are common for neighbors that are configured in different address families can be configured within the same peer session template. Peer session templates are created and configured in peer session configuration mode. Only general session commands can be configured in a peer session template. The following general session commands are supported by peer session templates: description disable-connected-check ebgp-multihop exit peer-session inherit peer-session local-as password remote-as shutdown timers translate-update update-source version General session commands can be configured once in a peer session template and then applied to many neighbors through the direct application of a peer session template or through indirect inheritance from a peer session template. The configuration of peer session templates simplifies the configuration of general session commands that are commonly applied to all neighbors within an autonomous system. Peer session templates support direct and indirect inheritance. A peer can be configured with only one peer session template at a time, and that peer session template can contain only one indirectly inherited peer session template. If you attempt to configure more than one inherit statement with a single peer session template, an error message will be displayed. This behavior allows a BGP neighbor to directly inherit only one session template and indirectly inherit up to seven additional peer session templates. This allows you to apply up to a maximum of eight peer session configurations to a neighbor: the configuration from the directly inherited peer session template and the configurations from up to seven indirectly inherited peer session templates. Inherited peer session configurations are evaluated first and applied starting with the last node in the branch and ending with the directly applied peer session template configuration at the source of the tree. The directly applied peer session template will have priority over inherited peer session template configurations. Any configuration statements that are duplicated in inherited peer session templates will be overwritten by the directly applied peer session template. So, if a general session command is reapplied with a different value, the subsequent value will have priority and overwrite the previous value that was configured in the indirectly inherited template. The following examples illustrate the use of this feature. In the following example, the general session command remote-as 1 is applied in the peer session template named SESSION-TEMPLATE-ONE: template peer-session SESSION-TEMPLATE-ONE 15

16 Information About Configuring a Basic BGP Network Peer Policy Templates remote-as 1 exit peer-session Peer session templates support only general session commands. BGP policy configuration commands that are configured only for a specific address family or NLRI configuration mode are configured with peer policy templates. Peer Policy Templates Peer policy templates are used to group and apply the configuration of commands that are applied within specific address families and NLRI configuration mode. Peer policy templates are created and configured in peer policy configuration mode. BGP policy commands that are configured for specific address families are configured in a peer policy template. The following BGP policy commands are supported by peer policy templates: advertisement-interval allowas-in as-override capability default-originate distribute-list dmzlink-bw exit-peer-policy filter-list inherit peer-policy maximum-prefix next-hop-self next-hop-unchanged prefix-list remove-private-as route-map route-reflector-client send-community send-label soft-reconfiguration unsuppress-map weight Peer policy templates are used to configure BGP policy commands that are configured for neighbors that belong to specific address families. Like peer session templates, peer policy templates are configured once and then applied to many neighbors through the direct application of a peer policy template or through inheritance from peer policy templates. The configuration of peer policy templates simplifies the configuration of BGP policy commands that are applied to all neighbors within an autonomous system. Like peer session templates, a peer policy template supports inheritance. However, there are minor differences. A directly applied peer policy template can directly or indirectly inherit configurations from up to seven peer policy templates. So, a total of eight peer policy templates can be applied to a neighbor or neighbor group. Inherited peer policy templates are configured with sequence numbers like route maps. An inherited peer policy template, like a route map, is evaluated starting with the inherit statement with the lowest sequence number and ending with the highest sequence number. However, there is a difference; a peer policy template will not collapse like a route map. Every sequence is evaluated, and if a BGP policy 16

17 BGP IPv6 Neighbor Activation Under the IPv4 Address Family How to Configure a Basic BGP Network command is reapplied with a different value, it will overwrite any previous value from a lower sequence number. The directly applied peer policy template and the inherit statement with the highest sequence number will always have priority and be applied last. Commands that are reapplied in subsequent peer templates will always overwrite the previous values. This behavior is designed to allow you to apply common policy configurations to large neighbor groups and specific policy configurations only to certain neighbors and neighbor groups without duplicating individual policy configuration commands. Peer policy templates support only policy configuration commands. BGP policy configuration commands that are configured only for specific address families are configured with peer policy templates. The configuration of peer policy templates simplifies and improves the flexibility of BGP configuration. A specific policy can be configured once and referenced many times. Because a peer policy supports up to eight levels of inheritance, very specific and very complex BGP policies can also be created. BGP IPv6 Neighbor Activation Under the IPv4 Address Family Prior to Cisco IOS Release 12.2(33)SRE4, by default, both IPv6 and IPv4 capability is exchanged with a BGP peer that has an IPv6 address. When an IPv6 peer is configured, that neighbor is automatically activated under the IPv4 unicast address family. Beginning with Cisco IOS Release 12.2(33)SRE4, when a new IPv6 neighbor is being configured, it is no longer automatically activated under the IPv4 address family. You can manually activate the IPv6 neighbor under the IPv4 address family if, for example, you have a dual stack environment and want to send IPv6 and IPv4 prefixes. If you do not want an existing IPv6 peer to be activated under the IPv4 address family, you can manually deactivate the peer with the no neighbor activate command. Until then, existing configurations that activate an IPv6 neighbor under the IPv4 unicast address family will continue to try to establish a session. How to Configure a Basic BGP Network Configuring a basic BGP network consists of a few required tasks and many optional tasks. A BGP routing process must be configured and BGP peers must be configured, preferably using the address family configuration model. If the BGP peers are part of a VPN network, the BGP peers must be configured using the IPv4 VRF address family task. The other tasks in the following list are optional: Configuring a BGP Routing Process, page 18 Configuring a BGP Peer, page 21 Configuring a BGP Routing Process and Peers Using 4-Byte Autonomous System Numbers, page 24 Modifying the Default Output and Regular Expression Match Format for 4-Byte Autonomous System Numbers, page 28 Configuring a BGP Peer for the IPv4 VRF Address Family, page 32 Customizing a BGP Peer, page 35 Removing BGP Configuration Commands Using a Redistribution, page 41 Monitoring and Maintaining Basic BGP, page 43 Aggregating Route Prefixes Using BGP, page 48 Originating BGP Routes, page 58 Configuring a BGP Peer Group, page 66 Configuring Peer Session Templates, page 68 17